High-voltage direct current (HVDC) connected wind power plants or other types of power plants as well, may be utilized to provide an electrical connection system between power generators and regional, national, or transnational transmission networks via high-voltage direct current (HVDC) converters & cables. Initial projects are under construction to provide HVDC subsea connections to offshore wind turbine generators. In these projects power is collected via medium-voltage alternating current (AC) subsea cables and is then stepped up to high-voltage AC at an offshore platform substation. An offshore HVDC converter rectifies the AC power to DC power and then transmits the DC power through an export cable to a land station converter. At the land station, the DC power may be inverted from DC power back to AC power to be fed into an electric grid. In such HVDC systems, voltage levels can be ±150 kV up to ±320 kV, or even higher.
In the above described system, the power systems of the wind turbine generators are configured conventionally, including an AC generator (for example permanent-magnet rotor synchronous machine) connected to a frequency converter (back-to-back AC/DC/AC). The converter rectifies generator power at variable frequency and variable voltage while outputting power to the collector grid at constant frequency and constant voltage, as occurs when connected to an AC transmission network. The voltage level is stepped up from a low voltage frequency converter level (690V, 50 Hz) to collector level (33 kV, 50 Hz) by a transformer of the wind turbine generator. Hence, the wind turbines generate AC power.
With such a conventional approach, the wind turbine generators experience an AC voltage as if connected directly to the AC transmission network on land since the offshore power collection grid is AC and has voltage levels of 33 kVac or even up to 66 kVac.
A problem of the prior art is that each wind turbine generator is equipped with a frequency converter, i.e. a complex electrical circuits, which may cause increased losses and reduced reliability.